Images are typically stored in a memory by representing tone values for each pixel of the original image. For a black and white image, the stored pixels represent the grayscale value corresponding to each pixel. For a color image, each color plane is stored as an array of pixels each representing the tone value for each pixel of the image in each respective color plane. For example, if each of the pixels of a black and white image is represented by an 8 bit digital word, then the tone value for a given image pixel may be one of 256 values between the black level and the white level.
Continuous tone images do not print well o most printing devices where typically the absence or presence of the ink on the paper is used to represent the printed image. In order to represent halftones (shades between the presence or absence of the printed ink), the original image is screened to produce a pattern, such as a variable size dots which appear to the human eye as a halftone image.
To prepare a photograph for printing, it is first necessary to perform the step of halftone screening, which converts the continuous gray shades of the original into dots of varying size and shape. Typically, these dots are arranged on a regular grid of approximately 100 dots per inch. This spatial frequency is known as the screen ruling. Thus, one square inch of the final printed photograph will be composed of approximately 10,000 dots.
Screening to produce halftone images is well known. The screen consists of an array of dots, or halftone cells, each of which represents one section of continuous tone in the original image as a single dot of variable size and shape. A halftone cell, in turn, consists of an array of smaller screen pixels, or samples, each having individual values against which the input pixels derived from the original image will be compared. The individual values of the smaller screen pixels, or samples, of the repeating halftone cell which form the variable dots is referred to herein as a spot function.
The halftone screening step consists of a screen pattern generating step, and a comparison step between the input image and the screen pattern. The screen is usually stored as a fairly small pattern that repeats itself or is repeatedly generated by programming. At any point where the original image is greater than the screen pattern, the output is marked. At any point where the image is not greater than the screen pattern, the output is not marked. In other words, if the value of the image pixel is greater than corresponding value of the screen cell, a mark is generated by the marking engine, whereas if the value of the image pixel is less or equal to the screen cell value, then no mark is generated by the marking engine, or vice versa. In this way, the final screened image, composed of dots, is produced. In color printing, there are four separate steps of halftone screening, one each for the cyan, magenta, yellow, and black inks.
Much prior art work has been devoted to producing screens which will produce good results and avoid artifacts in the final image. One such artifact to be avoided is the moire pattern which results from the interaction between the original image and the screen pattern. To counteract the tendency for moire patterns to result, color screens are angled at 0 (yellow), 15 (cyan), 75 (magenta) and 45 degrees (black). If these angles are adhered to precisely, as well as the screen ruling being precisely identical for all four planes, then optimum results (minimum moire) are achieved. Screening at these angles presents no special problem in photomechanical screening systems (achieved by simply rotating the photographic screen carrier). In digital screening devices and digital raster scan imaging devices there is some difficulty reproducing irrational numbers, such as the tangent 15 or tangent 75 degrees. Rational numbers can be represented as the ratio of two integers; irrational numbers are endless non-repeating decimals. Both rational tangent and irrational tangent digital screening systems are known.
In particular, many screening methods make use of a screen pattern cell, which can be a one dimensional strip, a band, or any two dimensional area, such as a square, that contains a section of the final screen The screening method will repeatedly generate the screen pattern cell, resulting in a final screen of the desired ruling and angle. The critical step in halftone screening is the generation of the screen pattern cell which will be repeated to form the final screen covering the entire input image area. Examples of prior art techniques include rational tangent (U.S. Pat. No. 4,149,194 to Holladay), rational supercell (European patent document 0 427 380 A2), irrational tangent (U.S. Pat. Nos. 4,499,489 to Gall et al, and 4,350,996 to Rosenfeld). In addition, three alternative screening techniques described in three copending patent applications by the inventor of the present invention, Ser. No. 753,893, filed Sep. 3, 1991, Ser. No. 768,135, filed Sep. 27, 1991, and Ser. No. 805,278, filed Dec. 10, 1991 may each be used to generate a suitable screen pattern cell.